Participants in this research study have prostate cancer and have had a prostatectomy or have been treated with radiation therapy. This type of cancer often has receptors which allow it to recognize vitamin D. Calcitriol is the active form of Vitamin D. It is an approved drug which is often used in patients with kidney disease who need extra Vitamin D. This use of Vitamin D has not been tested in humans, and therefore its use for cancer treatment is experimental. The purpose of this study is to determine if calcitriol will lower the PSA (prostate specific antigen) level. Researchers also want to find out if this treatment will stop or slow the growth of the tumor, to learn about the side effects of the drug and to examine the quality of life during the time patients participate in this research. Researchers will also examine patients' cells in a laboratory to look for changes calcitriol may cause in their tumor.